Benzene is considered to be environmentally hazardous. As a result, the State of California and the United States Environmental Protection Agency have instituted regulations to limit the amount of benzene which may be present in gasoline. As of January 2011, the US MSAT-2 (Mobile Source Air Toxics) regulation requires reduction of this annual average benzene content in gasoline to no greater than 0.62 volume %.
One known route for reducing the benzene content of gasoline is to selectively alkylate the benzene using a lower olefin. For example, Holtermann et al U.S. Pat. No. 5,149,894 describes a process for converting benzene to alkylated benzenes in a gasoline blend stock. The process involves contacting a benzene-containing gasoline blend stock with a C2 to C4 olefin stream in the presence of a catalyst containing the zeolite, SSZ-25, to produce an alkylated light hydrocarbon stream with reduced benzene content.
Cheng et al. U.S. Pat. No. 5,545,788 describes a process for the production of a more environmentally suitable gasoline by removing a substantial portion of benzene in gasoline by alkylation of reformate. The process involves alkylation using a light olefin feed at low temperature over the zeolite catalyst, MCM-49.
Umansky el al. U.S. Pat. No. 7,476,774 describes a process where light olefins including ethylene and propylene are extracted from refinery off-gases, such as from a catalytic cracking unit, into a light aromatic stream, such as a reformate containing benzene and other single ring aromatic compounds, which is then reacted with the light olefins to form a gasoline boiling range product containing alkylaromatics. The alkylation reaction is carried out in the liquid phase with a catalyst which preferably comprises a member of the MWW family of zeolites, such as MCM-22, using a fixed catalyst bed.
When benzene, such as that contained in a gasoline feed, is alkylated with an olefin in the liquid phase over a fixed bed catalyst, the reaction is highly exothermic, with an adiabatic temperature rise across the reactor. The maximum temperature rise across the reactor is set by practical limitations determined by the difference between the minimum reactor inlet temperature required for reaction, and the maximum outlet temperature which is deemed practical for the mechanical design of the reactor system.
To control the temperature rise, the quantity of olefin to each reactor stage is limited. In applications where the concentration of benzene in the refinery stream is relatively high, for example 20 volume % and above, the temperature limitation in an adiabatic process, according to current technology, requires that at least four stages in series are used, in order to be able to limit and distribute the olefin. The use of such a large number of reactors increases the cost of the plant substantially.
According to the present invention, it has now been found that the problem of the adiabatic temperature rise during the alkylation of a benzene-containing gasoline stream, such as a reformate or light naphtha, with an olefin alkylating agent can be reduced by diluting the feed to the alkylation reactor with a C5 to C10 paraffin stream. This problem of adiabatic temperature rise is more pronounced in a single phase system. i.e. vapor phase or liquid phase, as opposed to a mixed phase system including both a vapor phase and a liquid phase.